1. Field of the Invention
The present invention relates to fast-crystallizing polyester compositions comprising a linear aromatic polyester and a nucleating agent which is highly flame retardant.
2. Discussion of the Background
Polyethylene terephthalate (PET) is used in large amounts, particularly in the field of films and fibers, while its commercial development as a material for injection-molding has slackened off due to the long cycle times necessary to obtain dimensionally stable articles. The PET compositions utilized for injection-molding do not reach a high degree of crystallization, the molding times being equal, in comparison with other plastic materials. This is due to the fact that PET has a slower crystallization rate compared with other crystalline polymers and, therefore, it does not crystallize sufficiently in a mold at the low temperatures from 50.degree. to 95.degree. C. that are usually used for injection molding. Thus satisfactory molding products cannot be obtained, mainly because of changes in the volume of the article itself after molding.
However, it is known that the rate of crystallization of PET can be increased, which means that the crystallization temperature in the melt can be raised, by incorporating proper nucleating agents into the PET molding compositions. Many nucleating agents for the PET crystallization are well known in the art. For example, European Patent Application No. 31,201 describes the preparation of PET by reaction of terephthalic acid and ethylene glycol in the presence of sodium hydroxide, with in situ formation of the nucleating agent.
European Patent Application No. 21,648 describes the reaction of PET with a nucleating agent selected from ionizable metal salts of an organic compound having at least one acid proton. A useful class of aromatic organic acids are those of the formula R--(X).sub.n --(A).sub.m where A is an acid group selected from the carboxylic, sulphinic, phosphonic, phosphinic and phenol groups, m is an integer which is at least one, X is a direct link or is a --CH.dbd.CH-- or a --CH.sub.2 -- group, n is an integer between 1 and 4 and R is an aromatic structure consisting of single, fused or multiple aromatic rings. The preferred acid group is the carboxylic group. Preferred examples of such salts are the sodium salts of substituted benzoic acids which contain at least one nitro, halogen, hydroxyl, phenyl or oxyphenyl substituent. The phosphinic derivative is similar to compounds of the present invention but the prior art compounds have inferior flame retardant properties, especially in the absence of a halogen substituent.
European Patent Application No. 25,573 describes the nucleation of PET with a polyoxyalkylene containing at least one carboxylate group.
U.S. Pat. No. 3,761,450 describes lithium and/or sodium salts of aliphatic, cycloaliphatic, aromatic carboxylic acids, or heterocyclic polycarboxylic acids, containing up to 20 carbon atoms, as nucleating agents for PET.
British Pat. No. 2,013,014 describes sodium and potassium salts of selected organic polymers, containing pendent carboxylic groups, as agents capable of increasing the crystallization rate of PET in the composite materials.
U.S. Pat. No. 3,516,957 describes, as nucleating agents for PET, salts of alkali and alkaline-earth metals of organic acids having from 10 to 25 carbon atoms, such as sodium stearate.
Japanese Patent Publication No. 46/29977 describes sodium benzoate as a nucleating agent for PET.
U.S. Pat. No. 4,212,791 describes compositions comprising PET, an inert inorganic nucleating agent, an oligomer polyester crystallization promoter that does not contain carboxylates, and a copolyester-ether thermoplastic segmented elastomer.
European Patent Application No. 102,768 describes a method of nucleating PET by using a nucleating agent comprising an alkaline salt of a polyethylene terephtalate oligomer. The nucleating agent is first separately formed and then brought into contact with the PET to be nucleated.
Japanese Patent Publication No. 61/157556 describes an alkali metal salt of salicylic acid as a nucleating agent for PET and in Japanese Patent Publication No. 56/57825 is disclosed an alkali metal salt of phenol sulfonic acid as nucleating agent for PET.
However, in practice, nucleating agents which cause an appreciable increase in crystallization rates lead to undesirable molecular weight reductions; while the nucleating agents which do not lead to detrimental effects in this regard have little effect on the behavior of PET during crystallization.
U.S. Pat. No. 4,425,470 discloses attempts at nucleating PET without leading to drastic variations of the molecular weight, however, in this case the results are also not completely satisfactory.
European Patent Application No. 351,732 describes compositions comprising PET and an additive to improve its crystallization rate and moldability with less reduction in molecular weight and hydrolysis resistance. The additive is a metal salt of substituted phenols having at least one aromatic ring, at least one hydroxyl group bonded to the aromatic ring and at least one non-ionic substituent group bonded to the aromatic ring, in which the hydroxyl group is in the form of a metal salt.
U.S. Pat. No. 4,886,847 describes, as nucleating agents for PET, alkali or alkaline-earth salts of aryl- or cycloalkyl glycoxylic acids or derivatives thereof.
It is also known, from the U.S. Pat. No. 4,812,502, a flame-retarding additive for PET obtained by means of the oligomerization of a hydroxyalkylenephenylphosphinic acid or of a related alkyl ester thereof. The aforesaid additive imparts linear polyesters with self-extinguishing characteristics.
It has now been found that the above drawbacks can be overcome by using, as nucleating agents, compounds of formula (I) as described hereunder, containing phosphoric groups partly or wholly salified with alkali or alkaline-earth metals.